When .alpha.,.beta.-unsaturated aliphatic aldehydes such as acrolein or methacrolein are oxidized in the liquid phase with oxygen or an oxygen-containing gas such as air, peroxide (peroxy) compounds are co-produced along with the corresponding .alpha.,.beta.-unsaturated carboxylic acids. A mixture of products are obtained in the resulting oxidate solution formed by the oxidation. Based on the total weight, the oxidate solution will generally contain unreacted aldehyde of from 20 to 70 weight percent, and from 40 to 10 weight percent of the corresponding acrylic or methacrylic acid product as well as peroxide (peroxy) compounds of the unsaturated aldehyde feed materials and peroxide (peroxy) compounds of the unsaturated acid product and other by-products. The unsaturated peroxide (peroxy) compounds co-produced during the autoxidation of acrolein and methacrolein to acrylic acid and methacrylic acid would be peracrylic acid and acrolein monoperacrylate and permethacrylic acid and methacrolein monopermethacrylate respectively.
The present invention relates to a process for the selenium catalyzed conversion or decomposition of the above described unsaturated peroxide compounds coproduced by the autoxidation of the unsaturated aldehydes (acrolein or methacrolein). The selenium catalyzed decomposition may be carried out during the autoxidation of the aldehyde or after the autoxidation stage is completed. Employment of the selenium catalyst during the oxidation step of the aldehyde provides an in situ conversion of the intermediate peroxide compounds to the desired acid product. The oxidate product solution of the autoxidation of an aldehyde containing the mixture of products as hereinabove described may be treated according to the process of this invention; the process to catalytically decompose the peroxides being carried out after the aldehyde has been oxidized and the intermediate peroxide compounds formed. Conversion of the peroxide compounds according to this invention provides for a high selectivity to the acid and for the recovery of large percentages of the desired unsaturated carboxylic acid (acrylic or methacrylic) as well as a minimum amount of polymer formation, resulting in high overall yield of the acid from the particular original unsaturated aldehyde as compared to prior art processes, including straight thermal decomposition of peroxide compounds. Post catalytic decomposition of the oxidate, i.e., after completion of the oxidation of the aldehyde is preferred to in situ decomposition in that peroxide conversions are substantially complete and a higher selectivity to methacrylic or acrylic acid is obtained.
Various catalytic vapor phase processes have been described employing complex catalyst systems. Liquid phase reactions include the use of hydroperoxides, molecular oxygen including air in the presence of various solvents and various organic and inorganic metal compounds of silver, nickel, cobalt, manganese, copper, chromium and vanadium used singly, in combination and with other materials such as bromine compounds and chelates. Generally the oxidation of unsaturated aliphatic aldehydes to the corresponding acid in the liquid phase has been difficult due to polymerization of the unsaturated acids when formed and the co-production of various undesirable peroxides of the unsaturated acids and aldehydes formed during oxidation, resulting in low selectivity to and yield of the unsaturated acids.
U.S. Pat. No. 3,114,769 in an attempt to prevent polymeric by-products, describes a liquid phase process for the oxidation of methacrolein or acrolein to the corresponding acid and peroxide compounds in the presence of molecular oxygen and a small quantity of iodine. The products of the oxidation contained mixtures of unsaturated acids, and large amounts of both acid and aldehyde peroxides and unreacted aldehydes. After separation the peroxide products were separately decomposed to the acid by subjecting the oxidate containing peroxides to the catalytic effect of a protonic acid such as p-toluene sulfonic acid and an alcohol forming a hot solvent solution as is further described in U.S. Pat. No. 3,253,025.
In an article by William F. Brill and Fred Lister, Journal of Organic Chemistry, Vol. 26, pp. 565-569, 1961 the metal-salt catalyzed oxidation of methacrolein in acetic acid is described. The methacrolein goes to peroxide products, acid and major amounts of soluble polymer.
An article by Benjamin Phillips, et al, Journal of the American Chemical Society, Vol. 76, pp. 5982-5986, 1957 shows the preparation of peracetic acid by the autoxidation of acetaldehyde and with peracetic acid and acetaldehyde monoperacetate as intermediates. At temperatures above 20.degree. C. the acetaldehyde monoperacetate decomposes readily yielding acetic acid. Straight thermal decomposition of the .alpha.,.beta.-unsaturated aliphatic aldehyde peroxy intermediates, such as methacrolein monopermethacrylate and acrolein monoperacrylate does not selectively give high yield of the respective acrylic and methacrylic acids and in addition the rate of reaction is low.
To date no commercially successful process has been developed for the preparation of acrylic acid or methacrylic acid involving the autoxidation of the corresponding .alpha.,.beta.-unsaturated aldehyde and the conversion of the co-produced peroxide intermediate.
The acrylic and methacrylic acid products obtained by the process of this invention have many known commercial uses, particularly for the preparation of esters such as methyl methacrylate and as monomers for polymer formation.
A particular advantage of the process of the present invention is the discovery that catalytic amounts of selenium per se, organic and inorganic selenium compounds or mixtures thereof permit the respective peroxide (peroxy) compounds formed during oxidation of the unsaturated aldehyde to be selectively decomposed or converted to the acid, e.g., methacrolein monopermethacrylate to methacrylic acid providing an overall process advantage in the liquid phase autoxidation of the aldehyde to produce the desired unsaturated acid.